One of the major processes used in the foundry industry for making metal parts is sand casting. In sand casting, disposable foundry shapes (usually characterized as molds and cores) are made by shaping and curing a foundry binder system that is a mixture of sand and an organic or inorganic binder. The binder is used to strengthen the molds and cores.
Two of the major processes used in sand casting for making molds and cores are the no-bake process and the cold-box process. In the no-bake process, a liquid curing agent is mixed with an aggregate and shaped to produce a cured mold and/or core. In the cold-box process, a gaseous curing agent is passed through a compacted shaped mix to produce a cured mold and/or core. Polyurethane-forming binders, cured with a gaseous tertiary amine catalyst, are often used in the cold-box process to hold shaped foundry aggregate together as a mold or core. See for example U.S. Pat. No. 3,409,579. The polyurethane-forming binder system usually consists of a phenolic resin component and polyisocyanate component which are mixed with sand prior to compacting and curing to form a foundry binder system.
Among other things, the binder must have a low viscosity, be gel-free, remain stable under use conditions, and cure efficiently. The foundry binder system made by mixing sand with the binder must have adequate benchlife or the mix will not shape and cure properly. The cores and molds made with the binders must have adequate tensile strengths under normal and humid conditions, and release effectively from the pattern. Binders which meet all of these requirements are not easy to develop.
Ortho esters are known in the prior art to stabilize organic isocyanates. U.S. Pat. No. 3,535,359 (Chadwick) discloses that certain ortho-esters are capable of stabilizing a polyisocyanate against several different kinds of degradation, for instance moisture, and viscosity increases, even when only small amounts of ortho esters are used. The stabilized isocyanates are useful in the preparation of polyurethane foam, nonporous plastics including polyurethane castings such as gear wheels and the like, and coating compositions. Chadwick does not disclose the use of such polyisocyanates in foundry binders, foundry mixes, or the preparation of foundry shapes and metal castings.